PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 723 March 15, 2005
by Phillip F. Schewe, Ben Stein

DEGENERATE GAS STUCK IN OPTICAL LATTICE.  The forces that govern the
motions of macroscopic objects like planets and tennis balls are
complicated enough.  Forces among atoms at ultracold temperatures are even
more complicated.  In this regime atoms (pictured as being waves) spread
out so much that they overlap with neighboring atoms.
If the atoms are bosons (that is, if the total spin of each atom is an
integer) then they all fall into a single quantum state, namely a Bose
Einstein condensate (BEC).  If, however, the atoms are fermions (the total
spin is half-integral-valued), then quantum reality, in the form of the
Pauli exclusion principle, also decrees a special status: not a single
ensemble BEC state (all atoms having the same energy), but a state in which
none of the atoms has the same energy.  In this "Fermi
degenerate" state the atoms fill up all possible quantum energy
levels, one by one (or two by two, providing that the two atoms sharing a
level have opposite spins), until the last atom is accounted for.  (For the
first demonstration of a Fermi degenerate state in atoms, see
www.aip.org/pnu/1999/split/pnu447-1.htm.)  Now, physicists at the ETH lab
in Zurich have, for the first time, not only made a quantum degenerate
Fermi gas but have been able to load the atoms into the criss-cross
interstices of an optical lattice, an artificial 3D crystal in which atoms
are held in place by the electric fields of well-aimed laser beams.  Then,
by adjusting an external magnetic field, the pairs of atoms lodged in their
specified sites can be made to interact (courtesy of the "Feshbach
resonance") with a
varying strength.   According to Tilman Esslinger (41-1-633-2340,
esslinger{at}phys.ethz.ch), it is this ability to put atoms where you want
them in a crystal-like scaffolding, and then to make them interact with a
strength that you can control, that makes this setup so useful.  It might
be possible to test various condensed matter theories, such as those that
strive to explain high-temperature superconductivity, on a real physical
system.  (Kohl et al., Physical Review Letters, March 4; lab site,
www.quantumoptics.ethz.ch)
                                                
A PUZZLING SIGNAL IN RHIC EXPERIMENTS has now been explained by two
researchers as evidence for a primordial state of nuclear matter believed
to have accompanied a quark-gluon plasma or similarly exotic matter in the
early universe.  Colliding two beams of gold nuclei at Brookhaven's
Relativistic Heavy Ion Collider (RHIC) in New York, physicists have been
striving to make the quark-gluon plasma, a primordial soup of matter in
which quarks and gluons circulate freely.  However, the collision fireball
has been smaller and shorter-lived than expected, according to two RHIC
collaborations (STAR and PHENIX) of pions (the lightest form of
quark-antiquark pairs) coming out of the fireball.  The collaborations
employ the Hanbury-Brown-Twiss method, originally used  in astronomy to
measure the size of stars.  In the subatomic equivalent, spatially
separated detectors record pairs of pions emerging from the collision to
estimate the size of the fireball.  Now an experimentalist and a theorist,
both from the University of Washington, John G. Cramer (206-543-9194,
cramer{at}phys.washington.edu) and Gerald A. Miller (206-543-2995,
miller{at}phys.washington.edu), have teamed up for the first time to propose a
solution to this puzzle. Reporting independently of the RHIC
collaborations, they take into account the fact that the low-energy pions
produced inside the fireball act more like waves than classical,
billiard-ball-like particles; the pions' relatively long wavelengths tend
to overlap with other particles in the crowded fireball environment.  This
new quantum-mechanical analysis leads the researchers to conclude that a
primordial phenomenon has taken place inside the hot, dense RHIC fireballs.
 According to Miller and Cramer, the strong force is so powerful that the
pions are overcome by the attractive forces exerted by neighboring quarks
and anti-quarks. As a result, the pions act as nearly massless particles
inside the medium.  Such a situation is believed to have existed shortly
after the big bang, when the universe was extremely hot and dense. As the
pions work against the attraction to escape RHIC's primordial fireball,
they must convert some of their kinetic energy into mass, restoring their
lost weight.  But the pions' experience in the hot, dense environment
leaves its mark: the strong attractive force (and the absorption of some of
the pions in the collision) would make the fireball appear reduced in size
to the detectors that record the pions.  According to Miller, looking at
the fireball using pions is like looking through a distorted  lens: the
pions see the radius as about 7 fermi (fm), about the radius of an ordinary
gold nucleus, while the researchers deduce the true radius of the fireball
to be about 11.5 fm  (Cramer, Miller, Wu and Yoon, Phys Rev Lett, tent. 18
March 2005).

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